Adhesive tapes for sealing cartons are well known in the art. Three general types of tapes are typically used: paper tape, glass-reinforced water-activated tape, and pressure-sensitive tape. The different characteristics of these tapes make them suitable for certain uses but inadequate for others.
Paper tape is generally manufactured using a water-activated starch or animal-based adhesive thereon which is moistened immediately before the tape is applied to an article. This grade of water-activated adhesive tape is inexpensive, can be printed or marked, and lends itself to recycling, but it does not provide sufficient strength needed for certain applications. For example, when paper tape is used to seal and hold together a carton, the tape will tear or split when encountering torque forces.
Glass-reinforced water-activated tape typically has two plies of Kraft paper, laminated together by a hot melt polymeric adhesive, and sandwiched between the plies are lengthwise oriented or superposed arrays of fiberglass strands. The use of fiberglass strands makes the tape capable of withstanding much higher stresses and loading, but this kind of tape is significantly more expensive than simple paper tape. In addition, the laminated paper tape is relatively thick, given that it has the fiberglass strands sandwiched between its plies, so that the limited yardage in a roll of this tape gives rise to practical problems. For example, when the tape dispenser is in uninterrupted use in carton sealing operations, the tape roll is quickly exhausted, thereby making it necessary to frequently change the tape rolls.
Also, the hot melt polymeric adhesive used in glass-reinforced tape is not compatible with recycling operations, because the adhesive cannot be separated easily from the materials that are fed into the recycling equipment. Hence a standard, fiberglass-reinforced paper tape, when used to seal an otherwise recyclable article, usually renders it non-recyclable, unless the tape is first removed. Stripping the tape off, however, is a time-consuming and difficult process, for the gummed face of the tape is securely joined to the article, and if one attempts to strip this tape from the article, a residue of hot melt adhesive remains thereon and renders it unsuitable for recycling. In addition, when the tape is removed from the carton, it usually removes some of the corrugated paper as well, thus making it difficult to reuse the carton.
Furthermore, because the hot melt adhesive in the tape tends to build up on the cutting blade of the dispenser, it is necessary from time to time to shut down the dispenser to clean this blade.
The use of hot melt adhesive also has drawbacks in the manufacture of reinforced tapes in a conventional laminator. In manufacturing, two Kraft paper webs are continuously fed into the laminator, the surface of one web having been previously covered with a coating of hot melt adhesive. The hot melt adhesive must be maintained in a heated state while the laminator is in operation. Because this hot, flowable adhesive tends to drip over the operating mechanism of the laminator, it becomes necessary from time to time to shut down the laminator and use solvents to clean it. The use of volatile solvents may lead to adverse health and environmental effects, as well as additional costs in the process of recovering or disposing of the solvents. Also, as hot melt adhesives are sensitive to temperature changes, operation during cold temperature months can lead to partial cooling of the adhesive before application to the carton, with a weaker adhesive bond being achieved as a result.
A hot melt adhesive is one type of heat activatable adhesive. This type of adhesive is characterized as being inert and non-tacky at room temperature to facilitate handling of the tape, and is activated to a tacky or sticky state when heated to elevated temperatures. For example, U.S. Pat. No. 4,545,843 applies a heat activatable adhesive film to a porous substrate by heating the film and substrate to a temperature of between 200.degree. F. and 500.degree. F. to join the two together and form a laminate. A pressure sensitive adhesive is then applied to the film to provide an adhesive tape. Although the film in that patent is not used to adhere the substrate to a carton or similar object, the patent is cited as an illustration of the temperatures required to activate the adhesive. This type of adhesive is typically activated by heating 50.degree. C. above the Tg of the polymer in the adhesive, as shown in U.S. Pat. No. 4,880,683.
Pressure-sensitive tape is a another kind of tape commonly used by consumers and in commercial and industrial operations in which cartons and other articles are packed and sealed. The adhesives used for pressure-sensitive tapes are primarily hot melt adhesives that become pressure sensitive when coated upon a latex such as an acrylate. When a user cuts a strip of pressure-sensitive adhesive tape from a roll dispenser, this strip remains in a usable condition for an indefinite period, for however long the pressure-sensitive adhesive is exposed to the atmosphere, it continues to be sticky and the strip can be applied to a carton.
Unlike pressure-sensitive tape, a tape that is coated instead with a remoistenable adhesive, such as starch, must first be moistened with water. The water-moistened adhesive is then active for a limited "open-time," i.e., a limited time period during which the adhesive remains in a moistened state and in a condition to be applied to a carton or other article. With typical paper based sealing tape having a remoistenable adhesive coating on the paper, the open-time is from about 10 to 30 seconds, the more aggressive adhesives having shorter open-times. If the adhesive dries before the tape is applied to a box or carton, the tape is rendered inoperative and useless.
Problems with short open-times are avoided when pressure-sensitive adhesive tape is used, since pressure-sensitive adhesive remains sticky indefinitely. However, there are drawbacks to the stickiness, which results in handling problems when the tape twists, sticks to itself and thus renders long lengths of tape useless. In manual operation, such problems with pressure-sensitive tape result in undesirable waste and lowered efficiency.
Thus, a need exists for a tape that is strong, does not weaken when wet, does not require the use of solvents, is capable of being stripped from a carton in the same manner as standard room-temperature pressure sensitive tape, and unwinds freely without sticking to itself the way pressure-sensitive tape does. Such a tape should also be adaptable for use on existing automatic and semi-automatic machinery for packing and sealing cartons with only minor modifications of the equipment.